Preheat the oven to 400 degrees F (200 degrees C).
In a small bowl, whisk together the soy sauce, rice vinegar, sriracha, sesame oil, ginger, and garlic.
Place the salmon fillet in a baking dish and pour the sauce over it. Bake for 15-20 minutes, or until the salmon is cooked through.
While the salmon is baking, cook the sushi rice. In a medium saucepan, combine the sushi rice and water. Bring to a boil, then reduce heat and simmer, covered, for 15 minutes. Remove from heat and let stand for 5 minutes.
In a small bowl, whisk together the rice vinegar, sugar, and salt. Pour the mixture over the cooked rice and stir to combine.
To assemble the sushi bake, spread the sushi rice in a 9x13-inch baking dish. Flake the cooked salmon over the rice. Top with the avocado, cucumber, mayonnaise, sriracha, sesame seeds, and furikake seasoning (if using).
Bake for 10 minutes, or until heated through.
Serve immediately.